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“The rate of… (Class 12 Chemistry SQP 2026-27, Q18)

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ChemistryCBSE Class 12 · Sample paper, session 2026-27 · 2 marks · Very short answer

CBSE Class 12 Chemistry sample paper 2026-27: question 18

From the official CBSE sample question paper for 2026-27, section B, question 18, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q18 · 2 marks · Very short answerCompetency-basedChemical Kinetics

“The rate of reaction remains constant during the course of reaction.” Is this statement always true or is true under certain conditions? Reflect on the statement.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

Open one hint at a time and try again before the next.

Hint 1

Write the general rate law, rate = k[R]^n.

Hint 2

What happens to [R] as the reaction proceeds, and for which value of n does that not matter?

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. The statement is true only under a certain condition: when the reaction is zero order. For zero order, rate , so the rate does not depend on reactant concentration and stays constant as the reaction proceeds.

    1 mark

  2. It is not always true. For reactions of order 1, 2, etc., rate with . As reactants are used up, [R] falls, so the rate keeps decreasing during the course of the reaction.

    1 mark

Answer: True only for zero order reactions (rate = k). For other orders the rate decreases as concentration falls.

Where the marks are

  • States the statement holds for zero order reactions because rate is independent of concentration (1 mark)
  • Explains that for first/second order reactions the rate decreases as reactant concentration decreases (1 mark)

Revise the chapter

  • Chapter: Chemical Kinetics (notes, formulas, where marks are lost and every past question)
Practise this chapterBoard-style questions from Chemical Kinetics, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2026-27: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.